1. Field of the Invention
The present invention relates generally to an injection molding method employable for producing a desired molded article by injecting a molten resin into a die cavity. More particularly, the present invention relates to an injection molding method of the foregoing type which assures that a molded article having a large projected area can easily be produced by applying a low intensity of injection molding pressure to the die cavity without any occurrence of a malfunction such as warpage, shrinkage or the like.
2. Background Art
When an injection molding operation is performed by employing a conventional injection molding method, a pair of die halves constituting an injection molding die are mounted on the opposing surfaces of a stationary die board and a movable die board, and subsequently, one of the die halves on the movable die board is brought in tight contact with another one of the die halves on the stationary die board so as to allow the injection molding die to be held in the tightened state. At this time, the magnitude of die tightening force is determined depending on the intensity of pressure for injecting a molten resin, and moreover, the intensity of injection molding pressure to be applied to the injection molding die is determined depending on the projected area of a die cavity (i.e., a molded article). Thus, the larger the projected area of a molded article, the higher the injection molding pressure. This causes a large magnitude of die tightening force to be required for producing a molded article having a large projected area.
Conventionally, on the assumption that two die halves of the injection molding die are brought in tight contact with each other with a die tightening force which is assumed as a rated pressure corresponding to an intensity of injection molding pressure, each injection molding operation is achieved by way of a step of injecting and filling a molten resin in a die cavity of the injection molding die by applying a high intensity of pressure to the injection molding die until the die cavity is fully filled with the molten resin while the injection molding die halves are tightening with the predetermined die tightening force and a step of retaining the injection molding pressure in the die cavity after the die cavity is filled with the molten resin not only for the purpose of preventing reverse flow of the molten resin and reduction of the injection molding pressure but also for the purpose of compensating for the shrinkage of a molded article due to cooling of the injected molten resin. The intensity of the retaining pressure to be maintained in the die cavity during the pressure retaining step varies from molded article to molded article, and there arises an occasion that multi-staged control is executed for properly controlling the retaining pressure.
When the molten resin is injected in the injection molding die by applying a high intensity of injection molding pressure to the injection molding die, an inner stress is liable to remain in the molded article, and this tendency is remarkably recognizable with an molded article having a large projected area. In view of this fact, it is recommendable that each injection molding operation is performed with a low intensity of injection molding pressure. However, when the injection molding pressure is set to be lower than that corresponding to the die tightening force, there arises a malfunction that the injection molding die is warped due to a relatively large magnitude of die tightening force, causing an exhausting gap preliminarily formed along a parting plane between both the die halves to be closed. This often leads to the result that gas is irregularly exhausted from the injection molding die as the latter is filled with the molten resin, resulting in the injection molding die failing to be uniformly filled with the molten resin.
It is also contemplatable that each injection molding operation is performed depending on the contour of a molded article under a condition that both the intensity of injection molding pressure and the magnitude of die tightening force are reduced. In this case, it becomes difficult to retain the injection molding pressure in the injection molding die after completion of the injection of a molten resin due to reduction of the intensity of injection molding pressure. If the intensity of injection molding pressure is increased for the purpose of reliably retaining the injection molding pressure in the injection molding die, the latter is undesirably opened due to the unbalanced state between the injection molding pressure and the die tightening force, resulting in the injection molding pressure failing to be retained in the injection molding die.
In the case that a molded article has a small thickness, there arises a malfunction that warpage and/or shrinkage locally occurs on the molded article when the program goes to a step of retaining the injection molding pressure in the injection molding die while the intensity of pressure remaining in the die cavity is kept still high. An occurrence of the foregoing malfunction can be prevented to some extent by modifying the design of the contour of a molded article to be produced and a gate leading to the die cavity. However, in the case of a molded article having restrictions with respect to modificative designing of the contour and the gate, no measure can be taken for preventing the foregoing malfunction from arising.